Arsenic is known to be associated with both cancer and non-cancer health problems. This has recently been the focus of public attention due to the almost epidemic health problems of millions of people in Bangladesh and West Bengal, India, caused by As-contaminated groundwater (Kinniburgh, D. G. and Smedley, P. L., “Arsenic Contamination of Groundwater in Bangladesh, Vol. 2: Final Report,” BGS Technical Report WC/00/19, British Geological Survey, Keyworth, 2001; http://bgs.ac.uk/arsenic/bangladesh/reports.htm; Kinniburgh, D. G., Smedley, P. L., Davies, J., Milne, C. J., Gaus, I., Trafford, J. M., Burden, S., Ihtishamul Huq, S. M., Ahmad, N., and Ahmak, K. N., “The scale and causes of the groundwater arsenic problem in Bangladesh,” in Arsenic in Ground Water, Welch and Stollenwerk, eds, Kluwer, Boston, 2003 p 211—incorporated herein by reference in their entirety). Chronic exposure to As via drinking water causes skin, lung, bladder, prostate, and kidney cancer (National Research Council 1999; “Arsenic in Drinking Water” Washington, D.C., National Academy Press—incorporated herein by reference in its entirety). Recent evidence suggests that increased chronic exposure to As might also be associated with an increased risk of high blood pressure and diabetes.
To protect people against the effects of long-term exposure to As, the World Health Organization has set a provisional guideline concentration for drinking water of 10 ug/L (ppb) (WHO, “Guidelines for drinking water quality” 2nd Ed, Volume I Recommendations: World Health Organization, 1993, p 188—incorporated herein by reference in its entirety). The US EPA has proposed lowering the maximum contaminant level for As in drinking water from 50 ug/L to 10 ug/L (ppb). High concentrations of As tend to be found more in ground water than in surface water. Since more groundwater sources are used for public drinking water supplies or private wells, the As contamination in drinking water has become a serious worldwide issue. Ground water with a serious contaminant level of As can be found in many countries, including Bangladesh, Western India, Taiwan, Mongolia, Vietnam, Argentina, Chile, Mexico, and the United States (Matschullat, J., “Arsenic in the geosphere—a review”, Sci. Total, Environ., 249, 297-312, 2000—incorporated herein by reference in its entirety).
In the United States, half the population relies on ground water for drinking. There are many areas with a widespread high As concentration in ground water (Welch, A. H., Helsel, D. R., Focazio, M. J., and Watkins, S. A., “Arsenic in ground water supplies of the United States”, in Chappell, W. R., Abernathy, C. O. and Calderon, R. L., eds., Arsenic Exposure and Health Effects, Elsevier, Amsterdam, 1999, p. 416, incorporated herein by reference in its entirety). Arsenic concentrations up to 12 ppm (12,000 ppb) have been measured in ground water from a sandstone aquifer in the Fox River Valley in eastern Wisconsin. About 21% of private water supply wells in Outagamie and Winnebago counties in Wisconsin exceed 10 ppb and 4% of the wells exceed 50 ppb. In southeastern Michigan, about 70% of ground water samples taken from more than 100 wells, have an arsenic content of more than 10 ppb, with measured levels up to 220 ppb. In the Albuquerque Basin of central new Mexico, where more than 700,000 residents rely almost exclusively on ground water for drinking water supplies, the arsenic concentration in ground water underlying the basin has been detected in excess of 600 ppb, and concentrations exceeding 20 ppb are present across larger areas (Welch, A. H., Westjohn, D. B., Helsel, D. R., and Wanty, R. B., “Arsenic in ground water of the United States: Occurrence and geochemistry”, Ground Water, 38 (no. 4), 589-604, 2000—incorporated herein by reference in its entirety).
Reliable measurement of metal traces at the few ppb level is a challenging task. There are primarily two categories of methods for ultra-trace analysis; namely atomic absorption/emission spectroscopy (AAS, or AES) and inductively coupled plasma (ICP) based methods. In the AAS methods, a small sample (uL level) is placed into a high temperature graphite furnace where it is heated to the gas phase in the presence of a carrier gas. The attenuation of light of selected wavelengths is then measured to determine the amount of As. This is a very sensitive method and the detection limits are in the sub ppb range. However, the dynamic range of the technique is limited to 1-50 ppb and high concentration samples need to be diluted. Other disadvantages of AAS include high power consumption (˜6 kW) and requirement of inert ambient gas for the furnace. ICP based methods, including atomic emission spectrometry (ICP-AES) and mass spectrometry (ICP-MS), are powerful ultra-trace elemental analysis methods. The detection limit can go down to the part per trillion (ppt) range and these techniques have a very large dynamic range. However, ICP based methods are even more sophisticated than AAS methods. They require high vacuum, high power (˜6 kW), and plasma gas. Due to their degree of sophistication and requirement of extensive facilities, AAS and ICP methods are laboratory based methods and have great difficulty being used for field or on-line applications.
Other trace analysis techniques include electrochemical methods, such as potentiometric stripping analysis (PSA). These methods are based on electrochemical processes between electrodes and electrolyte solutions. In PSA measurements a reference is needed, as well as sophisticated sample preparation. They typically show poor reproducibility and are sensitive to the detailed chemical makeup of the sample. Consequently, electrochemical methods are not extensively used for laboratory analysis. Repeatability and reproducibility of these methods are major issues and make them difficult for field and on-line applications.